The binding of murine B and T lymphocytes to macrophages will be studied. The binding of a subpopulation of B cells will be studied in detail and the functional correlates of their binding determined. The subpopulation of T cells that regulates B cell binding will be identified. The T cell population that can bind to macrophages under antigen-free conditions will be identified. T cell that can bind to allogeneic or antigen pulsed macrophages will be phenotyped. The roles of lymphocyte-macrophage binding in lymphocyte differentiation will be studied.